This invention relates to an apparatus for feeding thread in a sewing machine. In a conventional sewing machine, a thread is fed by the reciprocating motion of a thread take-up lever. The length of thread fed is determined by the friction of a tension disc positioned in the thread pass. The tension disc is manually adjusted by an operator to match the sewing conditions.
Recent analysis of the sewing mechanism has found that the number of stitches that can be sewn depends on accurately controlling the length of thread fed into the needle. In other words, by sensing the thickness of a fabric to be sewn as well as sensing the pitch of the stitch, the length of thread that is necessary to be fed into the needle may be calculated, thus achieving automated sewing.
For example, an apparatus of this kind has already been proposed, in which the apparatus comprises a thread feed roller driven by a stepping motor, which controls the proper length of thread to be feed to the needle for every stitching motion.
In the known apparatus however, the length of thread tends to become greater than necessary due to the inertia of the apparatus. This tendency is emphasized when sewing at high speed. Furthermore, in sewing a thick fabric, the tension created in the thread is increased and therefore the stepping motor is required to have a greater capacity. This increases the manufacturing cost of the device, and makes it less competitive to market.